Tough coated hard particles (“TCHP” or EternAloy®) are a novel family of particulate materials. Conventionally, TCHP comprise at least one type of superhard, Geldart Class C or larger ceramic and/or refractory alloy core particles having extreme wear resistance, lubricity, and other properties which are individually coated with thin (e.g., nm) layers of a metal compound having a relatively high fracture toughness, such as WC or TaC. In conventional TCHP, an outer coating of a metal, such as Fe, Co or Ni, is provided around the individual particles. The combination of multiproperty alloys within the TCHP structure allows the combination of normally conflicting performance extremes, including, but not limited to toughness, abrasive wear resistance, chemical wear resistance, and light weight, at levels previously not attained from materials formed from sintered homogenous powders. TCHP materials are described in U.S. Pat. No. 6,372,346 to Toth, which is incorporated herein by reference. Methods for consolidating TCHP materials are described in U.S. Pre-Grant Publication No. 2005/0275143, which is also incorporated herein by reference.
The strength of a crystalline substance depends on atomic bonding and dislocation structure. Dislocations are linear atomic lattice defects that may be mobile, or may be pinned and immobile. Normally they are pinned and immobile. In a mixture of two atomically bonded crystalline materials that are combined to form a composite structure, there are upper and lower bound estimates to the elastic modulus of the composite as calculated by the rule of mixtures and the inverse rule of mixtures. Subjected to increasing load, the material deforms elastically until the dislocations in the grains begin to flow or slip, leading to the onset of permanent yielding and limiting useful strength. At particle sizes of approximately one micrometer and below, exceptionally high strengths can develop in such materials, due mainly to image dislocation stresses.
Typically there is a cylindrical strain field around each dislocation that extends outward into the surrounding lattice. Theoretically, this strain field around each dislocation must be balanced by opposing strain fields, otherwise the dislocation will move away from surfaces. When the crystal size is large compared to its strain field, no image stress is created around a dislocation unless it is at the crystal surface. In a sintered material wherein a plurality of crystalline particles are joined by a matrix material, the image stress matches the lower strength of the matrix, but for large crystals this is a trivial correction since most dislocations are not near a surface.
In submicron polycrystalline particles, the strain field may extend into neighboring grains, whose atomic lattice is most likely not aligned with that of the strain field of any neighboring grain. This balancing strain field outside the grain surface restrains movement of the dislocation, thus restraining yielding and increasing strength. As the size of the grains diminishes further, more dislocations are near surfaces and the strength further increases.
In conventional sintered TCHP, which include a core particle coated with an intermediate layer and, optionally, an outer layer of Fe, Ni, Co, or combinations thereof, the thickness of the intermediate layer is relatively thin. Although not precisely understood, it is believed that when the intermediate layer and the optional outer layer (if any) connecting the coated particles in conventional sintered TCHP are thin enough, the strain field actually passes through the outer layer material and into neighboring particles. This can result in the creation of high strength that is not controlled by the material between the TCHP particles (if any). In other words, the mechanical properties of conventional sintered TCHP can be independent of the properties of the outer layer phase, assuming it is crystalline and very thin.
While the transfer of strain fields in conventional sintered TCHP may result in certain improved properties, e.g., strength, it may adversely impact the toughness of the articles formed from such materials. As a result, articles formed from conventional sintered TCHP can exhibit very high strength, but may exhibit insufficient fracture toughness for some applications.
Thus, a need exists in the art for consolidated materials and articles that exhibit improved fracture toughness, relative to conventional sintered TCHP, while maintaining or substantially maintaining the hardness and/or other beneficial properties exhibited by conventional sintered TCHP. The consolidated materials and processes of the present disclosure achieve this goal, e.g., by dispersing TCHP in a tough matrix phase material, and/or by controlling the microstructure of the consolidated article so as to limit the transfer of strain fields between adjacent TCHP particles.